Numerical Simulation of Electric Currents through Insulating Materials

A numerical simulation of electric currents is carried out through solid dielectrics from a model based on the local polarization. A degradation of material was considered to be produced from one of the electrodes and we simulated it by admitting modifications in the chemical structure of the material, which implies an alteration in the value of the corresponding dipole moments that characterize the material, as well as the concentration of the latest. Different simulations are achieved according to the ratio of the degraded thickness and the temperature

Dielectric Characterization and Conduction Modelling of a Water Tree Degraded LDPE

Distribution of electric energy by extruded polymer insulated cables continues to be a subject of outstanding relevance in modern industrialized countries all over the world. Dielectric characterization, conduction modelling and finally diagnostics of polymeric insulations are necessary steps towards the development of reliable and less expensive robust technologies of electric power distribution. This paper is devoted to a detailed experimental / theoretical study of the conductive properties of LDPE affected by different levels of degradation by water trees. Water tree layers of different lengths were grown in accelerated conditions and were characterized by water tree kinetics, time-dependent permittivity and polarization current. The polarization current was found to obey a Curie-von Schweidler law whose parameters were used to characterize the effect of ageing time. A new conduction model that takes into account dipole interactions and was obtained from a two-wells Debye model is presented which allows us to give an interpretation of the effect of ageing. This laboratory study was intended to improve the characterization of service power cables aged by water trees

A Study on the Dependence of Water Tree Permittivity with Time

During the growth of water trees in the insulation of a cable the distribution of the electric field is modified because of the local change of the dielectric properties of the material. It results a local enhancement of the electric field which could increase the risk of breakdown. The key factor is the permittivity of the water tree and the aim of the work is to determine its possible values and, particularly, the law of its increase with time during the of the trees. The paper presents permittivity measurements in uniform field in MV and powe

Space Charge Studies on Mid-voltage Cable by Thermally Stimulated Depolarization Currents in the Melting Temperature Range.

In the present work, a XLPE mid-voltage cable from General Cable CO. has been studied by Thermally stimulated depolarization currents. Systematic measurements have been carried out in order to compare the conductive processes in this cable with previous results. Depolarization current as a function of thermal annealing, thermal history, polarizing field and polarizing time and temperature has been obtained. The results show the presence of a broad and complex heteropolar process between 60 and 120°C as expected. Annealing of the sample at temperatures above SOT develops an homopolar contribution associated to chemical components diffused from the cable semiconducting layers into the XLPE bulk. For annealing times of 60min at 140T and 2 days at 90°C, the homoplar current intensity reaches a maximum, decreasing and recovering the heteropolar sign with further annealing. Experiments performed with different polarizing times and temperatures show as well the presence of an homopolar contribution, overlapped to the heteropolar behavior, that increases continuously with polarizing time. These results indicate that conductive processes within the XLPE are probably responsible of homopolar charge injection

Comparative Study of Conductivity in Mid-Voltage Cable XLPE Insulation

It is well established that space charge conditions insulation lifetime. Because of this, space charge formation and relaxation processes characterization has become a very active research field, not only from an basic science point of view, but also in order to improve insulation performance in cable and other devices. In the case of mid-voltage power cable, crosslinked polyethylene is widely used as insulator. Service temperature of such cables is around 90°C, which in the case of the cable insulation studied is in the melting temperature range. In previous works it has been determined that at this temperature conductivity plays a relevant role in the space charge relaxation process, so that the characterization of conductive processes at service temperature may become important in order to enhance cable insulation performance. The aim of this work is to contribute to a better knowledge of the conductive properties of XLPE in both time and frequency domains

Electrooptical measurement system for the DC characterization of visible detectors for CMOS-compatible vision chips

Abstract—An electrooptical measurement system for the dc characterization of visible detectors for CMOS-compatible vision chips is presented, which can help designers to characterize these detectors. The measurement system has been designed to be versatile, fast, and easily expandable and used. Two different setups for the measurement of the spectral response and the optical dynamic range of the detectors are described in detail. Measurements of the spectral response are done with a fully computer-controlled setup, avoiding tedious and inaccurate measurements. A description of the different detectors available in a CMOS process is also given, together with the parameters affecting their response and a set of test structures which can be useful for the characterization of the detectors

Annealing Effect on the Conductivity of XLPE Insulation in Power Cable

Conductivity () in XLPE insulation of power cables annealed at 90 ºC at temperatures between 50 and 97 ºC has been measured. In all cases there is an initial increase in conductivity that develops a maximum and finally decreases for long annealing times. This maximum appears in the sample annealed 20 days when conductivity is measured at 50 ºC and shifts gradually to higher annealing times up to 40 days when the measurement is performed at 97 ºC. A linear behavior of ln() versus T -1/4 is observed, which implies that the transport mechanism is basically via thermally assisted hopping conduction. Infrared spectroscopy indicates that, during annealing, some chemical species diffuse from the semiconducting shields (SC) into the XLPE. Thermally stimulated depolarization currents technique (TSDC) and intensity-current measurements (I-V) point out as well the presence of this diffusion process that becomes less significant after long annealing times. The initial increase in is explained in terms of the increase in traps density due to the diffusion process from the SC shields. Long term decrease in is justified by the observed decrease of diffusion rate for long annealing times

Optical refractive index and static permittivity of mixed Zr–Si oxide thin films prepared by ion beam induced CVD

Mixed oxides ZrxSi1−xO2 (0bxb1) thin films have been prepared at room temperature by decomposition of (CH3CH2O)3SiH and Zr[OC (CH3)3]4 volatile precursors induced by mixtures of O2 + and Ar+ ions. The films were flat and amorphous independently of the Si/Zr ratio and did not present phase segregation of the pure single oxides (SiO2 and ZrO2). A 10–23 at.% of H and 1–5 at.% of C atoms remained incorporated in the films depending on the mixture ratio of the Si and Zr precursors and the composition of the bombarding gas used during the deposition process. These impurities are mainly forming hydroxyl and carboxylic groups. Optical refractive index and static permittivity of the films were determined by reflection NIR-Vis spectroscopy and C–V electrical characterization, respectively. It is found that the refractive index increases non-linearly from 1.45 to 2.10 as the Zr content in the thin films increases. The static permittivity also increases non-linearly from ∼4 for pure SiO2 to ∼15 for pure ZrO2. Optical and electrical characteristics of the films are justified by their impurity content and the available theories. © 2007 Elsevier B.V. All rights reserved

Effect of annealing on conductivity in XLPE mid-voltage cable insulation

A new study of the electrical conductivity of crosslinked polyethylene (XLPE) mid-voltage (MV) cable insulation is presented. Its main objective is to show the effect of annealing treatments on MV cables under actual service conditions. Complementary time domain (absorption/resorption currents) and frequency domain (dynamic electrical analysis) techniques are applied on different laboratory samples containing XLPE insulations: sections of XLPE insulated cable (with and without semiconducting screens) in the case of absorption/resorption currents and, in the case of dynamic electrical analysis, a thin ribbon obtained from the cable insulation by mechanical procedures. For annealing temperatures below a certain critical temperature, conductivity decreases both for XLPE cylinders (cable sections from which inner and outer semiconducting screens have been removed) and for real cables (sections of cable with semiconducting screens) but its value is smaller in the case of cables. If the annealing temperature is higher than the critical temperature, the behaviour of conductivity is more complex. In XLPE, cylinders conductivity initially decreases with the annealing time but after some annealing time it begins to increase, it passes over a maximum and eventually it decreases monotonically. In the case of real cable sections, conductivity grows, tending to a saturation value, which is noticeably higher than the corresponding value of the maximum obtained for XLPE cylinders. The experimental results are explained satisfactorily by means of the Mott equation that takes into account hopping conduction assisted both by temperature and electric field